Queries can be used to view, add or remove data in a system. A query is sent from a server to a system, where the system produces a necessary result responsive to the query, and returns the desired result back to the server. Queries can be used for retrieving data from one or more objects. In an example, queries are carried out in Structured Query Language (SQL). SQL queries for retrieving data from one or more data structures are initiated with a SELECT statement, which then can include multiple keywords and clauses (i.e. FROM, WHERE, ON). In a FROM clause, a JOIN sub-clause can be used to specify rules for combining records from two or more data structures (i.e. tables) to create a set of data or another data structure. There are different types of JOIN queries (i.e. INNER, OUTER, RIGHT and LEFT) that specify how the data structures are combined.
Systems can contain sets of data that share relationships with other sets of data. Related data can be queried through relationship queries, which allow multiple entities to be queried in a single request. A type of data relationship is the parent/child relationship, where one or more child entities correspond to a specific parent entity.